The invention relates generally to well fluid loss control systems and methods. Particularly, the invention relates to fluid loss control systems that can readily be removed when no longer needed.
In oil and gas drilling and servicing, fluid loss control agents are often used to minimize the loss of drilling or service fluids (including, for example, drilling muds and fracturing fluids) into the formation and to prevent damage to the formations by the leaked fluids. For example, specialized fluid loss control agents are typically employed to control fluid loss during drilling, completion, workover and stimulation operations. These fluid loss control agents are typically designed to control fluid losses for 48 hours or more depending on the applications. In gravel pack treatments where the fluid loss control agents must prevent fluid losses in high permeability formations for periods of 48 hours or more, such fluid loss control agents are referred to as kill pills or kill fluids. However, depending on the types of reagents used, these reagents themselves may be damaging to the formations if allowed to remain in the formations. Therefore, when the fluid loss control is no longer needed, these fluid loss control agents would have to be removed so that they do not damage the formations.
Providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss control agent. Conventional fluid loss control agents include oil-soluble resins, calcium carbonate, and graded salt fluid loss additives. These agents achieve their fluid loss control by having solids or particulates that form a filter-cake on the face of the formation to inhibit flow into and through the formation. However, these additive materials can cause severe damage to near-wellbore areas after their application. This damage can significantly reduce production levels if the formation permeability is not restored. In addition, these conventional fluid loss additives require long periods of clean up after their use, often by fluid circulation.
In addition to the particulate fluid loss control agents mentioned above, conventional gel agents such as viscoelastic surfactants and linear or metal-crosslinked polymers have also been used in controlling fluid loss. Hydroxyethylcelluloses (HEC) and various guars or modified guars are among the most commonly used polymers in fluid loss control agents. Normally, these polymers do not form rigid gels, but they can be made viscous by crosslinking with borate, zirconium, or other suitable metal ions. Examples of fluid control using such polymers can be found, for example, in M. E. Blauch et al., SPE 19752, “Fluid Loss Control Using Crosslinkable HEC in High-Permeability Offshore Flexure Trend Completions,” pages 465-476 (1989); U.S. Pat. No. 4,552,215, issued to Almond et al., SPE 29525, “A New Environmentally Safe Crosslinked Polymer for Fluid Loss Control,” pages 743-753 (1995), by R. C. Cole et al.; SPE 36676, “Development and Field Application of a New Fluid Loss Control Material,” pages 933-941 (1996), by P. D. Nguyen et al.; and U.S. Pat. No. 5,372,732, issued to Harris et al.
When these cross-linkable polymers (e.g., HEC or guars) are used as fluid loss control agents, they need to be removed after application to avoid damage to the formation. These agents are typically removed by circulating an external breaker, such as an acid. In addition to acids, U.S. Pat. No. 6,818,594, issued to Freeman et al., discloses the use of encapsulated enzymes to degrade such polymers. However, enzymes have limited use as breakers due to downhole temperature and pH restrictions.
While these conventional fluid loss control agents are useful, they may suffer from one or more of the following drawbacks: 1) Fluid loss control agents may lose efficiency over time, requiring the placement of additional control agents; 2) Removal of the fluid loss control agent often relies on circulating an external breaker, such as an acid; and 3) Fluid loss control agents may be damaging to the formation, especially if they are not completely removed.
When relying on circulating an external breaker to remove the fluid loss control agents, other issues may arise with the conventional fluid control technology, such as unreliable break times and complexity. While the conventional fluid loss control systems are sufficient for the purpose, there still exists a need for better fluid loss control systems. Ideally, good fluid loss control agents are stable for the durations needed under the downhole conditions and can be easily removed, preferably without the need to circulate an external breaker. In the case of kill pills, most preferably, it should be possible to remove fluid loss control systems on demand, so that there is no danger either of carrying out subsequent treatment too soon or of unnecessary waiting for the pill to break.